The mobile communication system is different from a wired communication system in that terminals constantly move. Accordingly, mobility of terminals should be taken into consideration when designing the mobile communication system and providing services through the system. That is, the mobile communication system needs to support the movement of a terminal from one region to another. When a terminal is moving away from a currently connected base station while approaching another base station, the network needs to switch a connection point of the terminal from the current base station to the other base station.
On the other hand, all user data should be transmitted within a limited time. In the case of a voice call, one voice signal needs to be sent from a user who speaks to another user who listens within a predetermined time. Otherwise, the call satisfaction of the listening user is reduced and it is difficult for them to converse normally. Also, Transmission Control Protocol (TCP) data needs to arrive at the receiving side within a predetermined time and the receiving side needs to acknowledge receipt of the data to the transmitting side within a predetermined time. Otherwise, the transmitting side will retransmit the TCP data, thereby wasting network resources. Accordingly, the mobile communication system needs to operate so that user data is transmitted within a predetermined time.
The mobile communication system also needs to reduce data loss in a radio interface as much as possible. For example, if one TCP packet is lost due to lower entities during transfer of the TCP packet, a TCP application determines that a congestion has occurred in the network and rapidly reduces the transmission rate of TCP packets. For example, when a packet loss occurs during a TCP application exchanges data at a rate of 100 Mbps, the TCP application may reduce the data transmission rate to almost 0 Mbps. This will rapidly reduce quality that the user experiences and make data transmission unstable. In the mobile communication system, the transmitting side retransmits data to minimize data loss when an ACK has not been received within a predetermined time or a NACK has been received for the data.